1. Field of the Invention
The invention relates to inspection of can seams by analyzing transmission of radiation, and in particular to a method of assessing the integrity of a can seam.
2. Related Art
Cans, such as those used for packaging food products, require adequate and uniform sealing. Typically, cans are three parts, a top, a bottom circular cover plate, and a cylindrical body section. Sealing of the top and bottom plates to the body section can be achieved by a double seam, wherein the edges of the plate and body are bent around one another to form (ideally) a hermetic seal. Such a double seam is shown schematically in cross-section in FIG. 1.
Can seam 100, having a seam width SW, is formed by an edge of top or bottom plate 103 bent around the edge of the body section 102. The body section bends to form a body hook BH, and the cover plate bends to form a cover hook CH. The can seam 100 is formed having a total external width W. Where the edges overlap, an overlap region OL is defined. An upper clearance UC and a lower clearance LC is defined by the width of the overlap region OL. Typically, the overlap region OL determines the sealing properties of the can. If an overlap region OL is not adequately formed, which may be due to a number of reasons including manufacturing defects, the sealing properties of the can seam can be compromised. The product within the can may then be at risk of putrefaction.
Since the integrity of can seams is of critical importance, methods of inspection have been developed. A conventional method of assessment is by visual observation of a cross-section of a seam, which would typically involve obtaining a cross-section of similar appearance to that shown in FIG. 1. Such a method can determine, for example, the degree of overlap, i.e. the length of the overlap region OL. An alternative method involves sectioning the seam across a plane parallel to the cover plate, in an attempt to reveal the extent of seal in the overlap region OL around the can seam circumference. These conventional methods are, however, time consuming, require destruction of the can seam, may be hazardous to the operator due to sharp cutting edges, and cannot reliably and automatically assess the integrity of a can seam. Such destructive methods also inevitably alter the state of the seam itself during preparation, through the release of internal stresses, and may therefore not produce an accurate picture of the state of the seam prior to the assessment being made.
Non-destructive methods of assessing can seam integrity have consequently been developed. One such method is disclosed in GB 2215834, which describes a method of inspection using X-ray analysis of a can seam. An X-ray beam is directed across the can seam to determine the length of the overlap region, through measuring a variation in transmitted radiation intensity across the width of the seam. An alternative disclosed method involves directing an X-ray beam tangentially across the can seam, and processing an image obtained from the analysis to obtain a cross-sectional view of the can seam. Both these methods aim to determine the quality of the seam through analysis of the overlap region, and in particular the width of the overlap region OL shown in FIG. 1. A measure of the whole of the can seam can be made by rotating the can relative to the X-ray beam and taking a succession of images.
U.S. Pat. No. 6,953,933 discloses further methods of determining the integrity of can seams by X-ray analysis, in which a measure of a size of a space between multiple layers of a can is obtained by scanning an X-ray beam across the can. An intensity distribution curve is obtained from which the dimensions of the can are determined.
Previous solutions to determining integrity of can seams, either through manual destructive methods or by non-destructive X-ray analysis methods, have certain drawbacks. One significant such drawback is that these methods have difficulty in determining the overall integrity of the can seam. If a sufficient width of overlap region exists around the seam, the integrity of the seam is largely determined by the uniformity of contact between the body section 102 and cover plate 103 in the overlap region OL. A sectional view, for example of the form shown in FIG. 1, or a plan view through the width of the can seam, is able to provide indications of the width of the overlap region OL, but not how uniform the contact between the body section 102 and cover plate 103 is around the circumference of the can seam. The term ‘free space’ is conventionally used in the canning industry to describe the amount of the overlap region OL in which there is not an intimate contact between the body section 102 and the cover plate 103. Increasing amounts of free space lead eventually to failure of the can seam. Currently free space can only be estimated by measuring across the seam, for example by determining the can seam width SW, and subtracting the known thickness of the metal sheet material. This is, however, not accurate. There is therefore a need for an improved method of determining free space.
It is an object of the invention to address one or more of the above mentioned problems.